One of the imaging modes of an atomic force microscope, which is a kind of scanning probe microscopes, is FM (Frequency Modulation) mode. In FM mode, a cantilever of the atomic force microscope apparatus is self-oscillated, and the interaction force between the cantilever and a sample is detected from the changes in the oscillation frequency. Then, the interaction force is imaged, or the surface shape of the sample is imaged by adjusting the distance between the cantilever and the sample such that the interaction force is kept constant.
FIG. 1(a) and FIG. 1(b) are graphs showing characteristics of a conventional scanning probe microscope.
FIG. 1(a) is a graph showing an example of a relationship between the interaction force and the distance between the cantilever and the sample. The cantilever has a particular mechanical resonant frequency which is determined by its own spring constant and a mass. When an external force as shown in FIG. 1(a) which varies with the distance between the cantilever and the sample is applied, an apparent spring constant is changed and therefore the resonant frequency is changed. FIG. 1(b) is a graph showing an example of a relationship between the resonant frequency and the distance between the cantilever and the sample.
FIG. 2 shows an example of a control system of a conventional FM mode atomic force microscope.
FIG. 2 shows a sample 101, a sample stage 102, an XYZ scanner 103, a cantilever 104 that measures characteristics of the sample 101, an oscillation detecting means 105 for detecting an oscillation of the cantilever 104, a detected signal waveform processing system 106 that receives a detected signal from the oscillation detecting means 105 to bandpass filter, stabilize amplitude and adjust phase, an FM detector 107 that is connected to the detected signal waveform processing system 106, a controller 108 that is connected to the FM detector 107, an oscillation exciting means 109 that is connected to the detected signal waveform processing system 106, an XY scanning and imaging system 110. The sample 101 can be scanned in the XYZ directions by using a Z-axis control signal from the controller 108 and an XY scanning signal from the XY scanning and imaging system 110.
In other words, a detected signal of an oscillation of the cantilever 104 is amplified, stabilized in amplitude, and phase-adjusted if necessary by the detected signal waveform processing system 106. Then the signal is fed back to the oscillation exciting means 109, and the cantilever 104 is self-oscillated at the resonant frequency. The resonant frequency of the cantilever 104 and therefore the interaction force between the cantilever 104 and the sample 101 can be obtained by detecting the frequency of the self-excited oscillation by the FM detector 107.
An interaction force image can be obtained by XY scanning the sample 101 according to an XY scanning signal from the XY scanning and imaging system 110 while detecting the interaction force as described above, and imaging the interaction force at each XY coordinate point. Furthermore, an image of the surface shape of the sample 101 can also be obtained by XY scanning the sample 101 while controlling the distance between the cantilever 104 and the sample 101, which is the position of the Z-axis, according to the Z-axis control signal from the controller 108 such that the interaction force is kept constant.
The feedback loop (self-excitation loop) which generates a self-excited oscillation may include frequency conversion process. This system is called a super heterodyne system. The super heterodyne system can be combined with a PLL to stabilize the oscillation.
Phase feedback system used in sample imaging apparatus is described in Applied Surface Science 157 (2000), pp. 332-336. The phase feedback system will be hereinafter described in detail.
Oscillation of a cantilever is described in WO 02/103328.
A probe and probe microscope apparatus are described in WO 2005/015570.